1. Field of the Invention
This invention relates to display arrangements for head-up display systems, more especially head-up display systems for use in vehicles, more especially aircraft.
2. Description of Related Art
In recent years there have been proposed various forms of such display arrangement using one or more tuned reflective optical films for the combiner of the arrangement, that is to say, for the element of the arrangement through which the observer views the distant scene and which serves to reflect light representing an image to be displayed to the observer, thereby to superimpose the observer's view of the image on his view of the distant scene.
By "a tuned reflective optical film" is meant an optical film or similar optical element having the property of reflecting efficiently light at particular combinations of wavelength and incidence angle. The film is described as tuned to these combinations of wavelength and incidence angle. Light which does not satisfy the tuned condition is transmitted by the film with little reflection loss.
In one such arrangement, described in United Kingdom Patent Specification No. 2042206 the combiner has first and second tuned reflective optical films each of which intercepts the line of sight of the observer through the combiner and which coverage towards one another so as to define a tapering space; the angular relationship of the films with one another and with the display surface of a projector unit producing the image to be displayed being such that light within a certain narrow waveband from the display surface is, in turn, reflected at the second coating, reflected at the first coating and transmitted through the second coating to the observer's viewing position.
Such an arrangement has the advantage of being compact while providing a large field of view for the observer but has a disadvantage in that the user views the distant scene through two tuned reflective optical films. Inevitably there is a loss of `real world` brightness with such an arrangement.